J . A m . Chem. SOC.1985, 107, 1299-1308 dioxide had evaporated, 25 mL of saturated aqueous sodium bisulfate and 25 mL of ether were added followed by 3 mL of concentrated hydrochloric acid or enough to reach pH 2. The layers were separated, the aqueous layer reextracted with 15 mL of ether and the combined organic layers dried (Na,SO,). The organic layer was then cooled to 0 OC and stirred while an ethereal solution of diazomethane was added portionwise until TLC showed complete reaction. Removal of solvent by atmospheric distillation followed by flash chromatography (5% ether in pentane), reconcentration, and distillation (bulb-to-bulb, 80 O C , 0.02 mmHg) yielded 1I O mg (0.99 mmol 66%) of ester from the ketone. R t 0.22 (4:l hexane/ethyl acetate) for the acid, 0.55 for the ester. The following data were obtained on a sample containing 20% of the bridgehead olefin and a 2:l ratio of methyl epimers: IH N M R (270 MHz) 6.68 (m,1 H), 4.98 (m,0.13H),4.93(q,J=2,0.07H),4.88(m,0.13H),4.86(q,J=2, 0.07 H), 4.79 (m, 0.36 H), 4.76 (m,0.44 H), 4.69 (m,0.8 H), 3.73 (s, 1.6 H), 3.72 (s, 1.4 H), 3.35 (m,2 H), 2.9-2.6 (m, 2 H), 2.3-2.1 (m,3 H), 1.14 (d, J = 7,0.21 H), 1.09 (d, J = 7, 1.32 H), 0.96 (d, J = 7, 1.08 H), 0.86 (d, J = 8, 0.39 H); IR (CHC13) 2962, 1705, 1636, 1438, 1275, 1100, 900, 632; MS 192 (54), 161 (13), 160 (19), 133 (62), 132 (50), 117 (22), 105 (17), 91 (26). Anal. Calcd for ClzH,,Oz: C, 74.96; H, 8.39; MW, 192.1150. Found: C, 74.79; H , 8.39; MW, 192.1151. Preparation of 2-Carbomethoxy-6-methyl-7-methylenebicyclo[3.3.0]oct-7-ene (20). n-Butyllithium (0.56 mL, 0.97 mmol of 1.74 M hexane solution) and HMPA (174 mg, 0.97 mmol) were added to a -78 O C solution of 0.85 mL of T H F and 98 mg of diisopropylamins (0.97 mmol). After allowing the solution to warm to -30 O C until the solution became homogeneous, the flask was recooled to -78 O C and the ester (125 mg, 0.65 mmol) in 0.94 mL of T H F was added dropwise over 10 min. The light yellow solution was allowed to stir for an additional 10 min, then cannulated onto a biphasic ether/saturated aqueous sodium bisulfate solution (5 mL each), to which was added 15 mL of pentane. After separation, the organic layer was washed with aqueous sodium bicarbonate (1 X 2 mL), saturated aqueous cupric sulfate (2 X 3 mL), water (1 X 2 mL), and dried (Na,SO,). Solvent removal via atmospheric distillation followed by bulb-to-bulb distillation [SO OC (0.01 mmHg)] yielded 101 mg (0.45 mmol, 69%) of product. IH N M R (270 MHz) 6.42 (bs, 0.04 H), 6.35 (bs, 0.08 H), 5.90-5.78 (m, 0.88 H), 5.70-5.60 (m, 0.88 H), 4.80-4.60 (m, 2.0 H); four singlets, 3.73, 3.71, 3.70, 3.69 (3 H), 3.01-1.81 ( m , 6 H ) , 1 . 1 4 ( d , J = 6 . 8 H z , 0 . 8 8 H ) , 1 . 1 3 ( d , J = 6 . 8 H z , 1.76 H), 0.95 (d, J = 6.8 Hz, 0.12 H), 0.93 (d, J = 6.8 Hz, 0.24 H). IR (CHC13): 1729, 1460, 1440 cm-l. These spectral data represent a sample with 12% of the olefin isomer and a 2:1 ratio of methyl epimers. Preparation of 1-0-Methyldehydrologanin Aglucon (2lb). A dry , bubbled into a -78 OC solution of 39.5 mg (0.18 stream of 0 3 / 0was
1299
mmol, 0.21 mmol of combined olefin isomers) of 20 in 4 mL of methylene chloride until a blue color appeared. After removing the excess ozone with a stream of nitrogen, the solvent was removed in vacuo followed by 2 min on a vacuum pump. Reduction was then accomplished by dissolving the bis(ozonide) in 3 mL of acetic acid and adding zinc dust (80 mg, 1.23 mm) in one portion followed by stirring at room temperature for 2 h. The solution was then added to a separatory funnel containing 25 mL each of ether and saturated aqueous sodium bicarbonate. Solid sodium bicarbonate was added until the acetic acid was decomposed. The aqueous layer was extracted further with ether (3 X 25 mL). The combined organic layers were dried (MgSO,) and concentrated to yield crude hydroxyacetal. The oil was dissolved in 3 mL of methanol containing TsOH (17 mg, 0.1 mmol) for 2 days at room temperature. Sodium methoxide (20 mg, 0.37 mm) was then added to the flask in a glovebag and the flask was kept at 2 OC for 18 h. The reaction was neutralized with acetic acid, concentrated in vacuo, extracted with ether, washed with aqueous sodium bicarbonate, dried (MgSO,), and plated (6:4 hexane/ ethyl acetate) to yield 12.1 mg (0.05 mm, 28%) of quite pure material which was identical with material obtained from degradation of natural loganin: Rf0.30 (6:4 hexane/ethyl acetate) for hydroxyacetal, 0.50 for methoxyacetal. Evidently the deconjugated olefin isomer forms a keto acid upon ozonolysis which is lost in the basic workup.
Acknowledgment. We thank the National Science Foundation and t h e National Institutes of Health, General Medical Sciences, for their generous support of o u r programs. Registry No. exo-6, 79348-44-0; endo-6, 79390-46-8; 7a, 79348-40-6; 7b, 94235-18-4; 7c, 94235-19-5; 9, 91495-68-0; 10,94235-20-8; (2)-11, 94235-21-9; (E)-11, 94235-22-0; exo-18, 94235-23-1; endo-18,9423524-2; 19 (isomer l ) , 94235-25-3; 19 (isomer 2), 94235-26-4; 20, 9423527-5; 20 (bis(ozonide)), 94235-31-1; 21a, 86342-78-1; 2lb, 50427-62-8; 22, 94235-28-6; 23 (isomer l ) , 94292-76-9; 23 (isomer 2), 94292-77-0; 24 (isomer I), 94292-78-1; 24 (isomer 2), 94292-79-2; 3-methyl-3-buten-2-01, 10473-14-0; 2-(trimethylsiloxy)-3-[(trimethylsilyl)methyl]-3butene, 94235-29-7; 2-(trimethylsiloxy)-3-[(trimethylsilyl)methyl]-4(trimethylsilyl)-3-butene, 94235-30-0; 3- [(trimethylsilyl)methyl]-3-b~ten-2-01, 79348-42-8; 2-[(trimethylsilyl)methyl]propenal, 56407-82-0; 2-cyclopentenone, 930-30-3; [(2,4,6-triisopropylphenyl)sulfonyl]hydrazine, 39085-59-1.
Supplementary Material Available: G e n e r a l experimental procedures (2 pages). Ordering information is given o n a n y current masthead page.
Exciton Approach to the Optical Activity of C3-Cyclot river at rylene Derivatives Josette Canceill,+ AndrC Collet,*+Jacqueline Gabard,+ Giovanni Gottarelli,* and Gian Piero Spadat Contribution f r o m the CollPge de France, Chimie des Interactions MolPculaires,s 1 1 , place Marcelin Berthelot, 75005 Paris, France, and the Universitci di Bologna, Istituto di Scienze Chimiche, via S. Donato 15, 40127 Bologna, Italy. Received July 2, 1984 Abstract: The circular dichroism of chiral C3-cyclotriveratrylenes 2-12 in which the substitution patterns correspond to various combinations of RI and R2= H , OH, 0-,and OAc, and 0-alkyl groups has been analyzed in light of the exciton theory, using the concept of spectroscopic moments. From the observed signs and intensities of the BZucouplets, a self-consistent set of polarization angles for this transition in the OH/O-alkyl ortho-disubstituted derivatives has been established. The spectroscopic moments of these substituents have been shown to increase on going from the bulkiest (O-i-C,H,) to the smallest ( O H ) group, very likely as a consequence of different equilibria between planar and nonplanar conformers. Finally, the experimental Bzu and BIucouplets have been satisfactorily reproduced in most of the cases studied by calculations based on the exciton approximation, with limited p-a configuration interaction.
Cyclotriveratrylene (1) a n d its analogues devoid of bulky substituents o r t h o t o t h e nine-membered ring are rigid, coneshaped molecules t h a t exhibit stable optical activity a t ambient temperature when t h e achiral C,,s y m m e t r y of t h e parent com-
'Coll6ge de France.
'Universitl di Bologna. SGroupe de recherche du C.N.R.S. No. 20.
0002-7863/85/1507-1299$01.50/0
pound is destroyed by appropriate substitution (e.g., when R l # RJ.' W i t h t h e exception of a C,-monobenzyl ether which was partially resolved in 1966 by Liittringhaus? all the optically active (1) Collet, A. In "Inclusion Compounds"; Atwood, J. L., Ed.; Academic Press: London, 1984; Vol. 11, Chapter 4 and references therein. (2) Luttringhaus, A,; Peters, K. C. Angew. Chem. 1966, 78, 603; Angew. Chem., Int. Ed. Engl. 1966, 5 , 593-594.
0 1985 American Chemical Society
1300 J . Am. Chem. SOC.,Vol. 107, No. 5, 1985
Canceill et al.
Schcme 1
Ha
RO-CHzOH
OR 0
R2
Ri R2
1-)-17
(t)-
and 1-j-2
$$lH
optical
OCH3
2
OH
3
OH
U
resolution
4
OCOCHj
derivatives of 1 known to date belong to the C3and D3symmetry
5
OCOCHj
group^.^-^
6
OCD3
7
OCH3
8
OiC3H7
9
OiC3H7
As in the well-known case of the C2-biaryls,* the chiroptical properties of C3-cyclotriveratrylenes merely originate from through-space interactions of equivalent, intrinsically achiral aromatic subunits, giving rise to well-characterized exciton circular dichroism (CD) spectra. More precisely, the optical activity in these compounds results from more or less important deviations of the long- and short-axis polarized components of the transitions of the phenyl rings from the symmetrical positions, as a consequence of the presence of two substituents R , and R2 having different propertiesg Due to the favorable (and rigid) geometry and to the relatively strong dipole strengths of the transitions in this system, even a very small distortion yields intense exciton CD, which in turn provides useful information as to the substituent effects on the properties of the aromatic chromophore; this system, as we have shown, is sensitive enough to evidence isotope substitution effects (e.g., in 6 and 13).I0-l2 In the present article, we analyze in the light of the exciton mechanism the CD properties of a series of C3-cyclotriveratrylenes, 2-12, in which the substitution patterns correspond to various combinations of R, and R, = H, OH, 0-, 0-alkyl, and 0-acyl groups. We also describe several new procedures that we have developed since our previous reports3s5for the optical resolution of C3-cyclotriguaiacylene (2) as well as for the synthesis and resolution of the ethoxy analogue 3. The synthesis and C D spectrum of isotopically chiral C3cyclotribenzylene-d3 (13) have been reported ~ e p a r a t e l y , ~and J~ the C D of D3-bis(cyclotriveratrylenyl)6 and related compounds bearing two cyclotriveratrylene fragments will be discussed in a forthcoming paper. Results Syntheses and Absolute Configurations. The absolute configurations of the C3-cyclotriveratrylenes 2-12, shown on the stereoformulas with the corresponding M o r P descriptors, are based on the previous X-ray determination of the structure of 22, bearing a chiral group R2 of known stereochemi~try.~ This compound was one of the diastereomers formed by acid-catalyzed trimerization of (R)-(+)-15; after appropriate transformations it afforded (-1-9 and (-)-3, which on methylation gave (-)-7. On the other hand, (3) Collet, A,; Jacques, J. Tetrahedron Lett. 1978, 1265-1268. (4) Collet, A,; Gabard, J. J . Urg. Chem. 1980, 45, 5400-5401. (5) Collet, A.; Gabard, J.; Jacques, J.; Cesario, M.; Guilhem, J.; Pascard, C. J . Chem. Soc., Perkin Trans. 1 1981, 1630-1638. (6) Gabard, J.; Collet, A. J . Chem. SOC.,Chem. Commun. 1981, 1 1 37-1 139. (7) Canceill, J.; Collet, A. J . Chem. Soc., Chem. Commun. 1983, 1145-1 147. ( 8 ) Mason, S. F.; Seal, R. H.; Roberts, D. R. Tetrahedron 1974, 30, 1671-1682. (9) Collet, A.; Gottarelli, G. J . Am. Chem. SOC.1981, 103, 204-205. (10) Collet, A.; Gottarelli, G. J . Am. Chem. SOC.1981, 103, 5912-5913. ( I 1) Canceill, J.; Gabard, J.; Collet, A. J . Chem. SOC.,Chem. Commun. 1983. 122-123. ~ . ~-~ , .-. (12) Canceill, J.; Collet, A.; Gottarelli, G. J . Am. Chem. SOC.1984, 106, 5997-6003. ~
1
10
H
11
H
12
H
13
H
0 II
$H3 4
q C H 2 O H R = CH3
HO-C-C-0 H
OR
R=C2H5
(+)-15
trimerization3 of (R)-(+)-14 likewise gave, inter alia, diastereomer 23 which by a multistep sequence was converted into cyclotriguaiacylene, (+)-2. Since on ethylation the latter also gave (-)-7, its absolute configuration was accordingly established as (M)-(+) or (E')-(-). Soon after this work was published we found"J2 a more straightforward access to multigram quantities of the key triphenol (f)-2, by trimerization of the phenolic allyl ether of vanillyl alcohol (18) to the derivative 19, which in turn was converted into 2 in high yield (Scheme I). We have now extended this route to the preparation of optically active 2, by resolving the diastereomer mixture of 24 and 25 obtained from the racemate and (-)-acamphanic acid (17). The diastereomeric triesters were completely separated by chromatography and crystallization, and their reductive cleavage gave back triphenols (+)- and (-)-2 which exhibited the rotations indicated in Table I. Several preparations, using either the above procedure or the earlier ~ynthesis,~ afforded enantiomers, having [(Y]D 271' (*4%) in chloroform. Recrystallization of partially resolved samples (e.g., [(Y]D 205') also raised the rotation to the same value, which therefore very likely represents the maximum rotation (*4%) of 2. We also attempted to apply the sequence of Scheme I to the synthesis of 3. However, the phenolic allyl ether 20 did not trimerize satisfactorily under the conditions in which 18 gave 19,13 (13) The phenolic allyl ether of ethylvanillyl alcohol (20) (mp 32 "C) on reaction with 65% perchloric acid in methanol or acetic acid solution invariably afforded trimer 21 (mp 116 "C) in ca. 15% yield, instead of 50% for the similar conversion of 18 into 19.'* The reasons for this difference are not entirely clear; as a matter of fact, acid-catalyzed condensation of 3,4-disubstituted benzyl alcohols only rarely afford cyclotriveratrylenes in good yield. These reactions are extremely sensitive to the nature of the substituents as well as the experimental conditions. However, it is likely that the solubility of the trimer is one of the factors determining the yield. When the latter crystallizes off during the course of the reaction (this is the case of 19). it is protected from side reactions which can occur under the strongly acidic conditions employed, and the yield is eventually better.
J . Am. Chem. Soc., Vol. 107, No. 5, 1985 1301
C3-Cyclotriveratrylene Derivatives Scheme I1
7
I
R1
22
OC2H5
X
23
X
OCH3
t
74O
24
OCH3
Y
-
52O
25
Y
OCH3
t
47O
26
OC2H5
2
f
37O
27
2
OC2H5
t
99O
Q
-16.1’
optical
resolution
X
HO
OC~HS
Scheme 111. Polarization Direction 0 of the B,, Transition Dipoles as a Function of the Spectroscopic Moments of the Substituents R , and R Z a
0 YCO . I
H w 6-C H3
C02CH3 Y 2 X and this circumstances led us to adopt the alternative route summarized in Scheme 11. Ethylation of the phenol group of vanillyl alcohol furnished the starting compound 30, which on reaction with 65% perchloric acid gave (f)-7 in ca. 50%isolated yield. Cleavage of the methyl ethers in 7 could then be effected in a totally selective manner, by reaction with lithium diphenylph~sphide,’~ providing the desired triphenol (f)-3 in 78% yield. Optical resolution of 3 was then achieved by converting this compound into the mixture of triesters 26 and 27, by reaction with (R)-(+)-2-phenoxypropionic acid (16).15 Chromatographic separation followed by reductive cleavage of each pure diastereomer finally afforded crystalline enantiomers of 3, having rotations (Table I) 12% greater than previously reportedSfor a glassy sample of (-)-3 obtained from 22 by the earlier m e t h ~ d . ~ Finally, acetates 4 and 5 were obtained from the corresponding triphenols 2 and 3 by reaction with acetyl chloride in pyridine at 0-20 “C, and reaction of the sodium salt of 2 with CD31 or (CH3),CHBr in HMPA at 20 “ C provided 6 and 8. The synthesis of 10-13 from 2 was effected as described in detail elsewhere.12 The specific rotations assembled in Table I correspond to the values observed for recrystallized samples and very likely represent the maximum rotations (*-5%) of these substances.12 As previously noted,’J2 chiral cyclotriveratrylenes racemize on heating, via crown inversion, over a barrier of 110-1 15 kJ/mol. The rate constant for the inversion process at room temperature is about 10-7-10-8 SKI,which corresponds to a 1% rotation decrease over 12-48 h in solution. These compounds can, therefore, be considered optically stable and conformationally homogeneous’ in the conditions in which the CD measurements discussed below were recorded. Exciton Approach to the Optical Activity of 2-12. In the exciton approximation,16 the wave functions of cyclotriveratrylene de(14) (a) Ireland, R. E.; Walba, D. M . Org. Synth. 1977, 56, 44-48. (b) Vedejs, E.: Fuchs, P. L. J . Am. Chem. Sor. 1973, 95, 822-825. (15) Fourneau, E.; Sandulesco, G. Bull. SOC.Chim.Fr. 1922, 31,988-993. Fredga, A,; Matell, M. Ark. Kemi 1952, 4, 325-330. Sjoberg, B.; Sjoberg, S . Ark. Kemi 1964,22,447-450. The (R)-(+)-2-phenoxypropionic acid that we used had [alZso+39.7O in absolute ethanol and was enantiomerically pure. This resolving agent can also be prepared by reaction of the O-p-toluenesulfonate of S-(-)-ethyl lactate with phenol and potassium carbonate in acetonitrile (5-h reflux), followed by alkaline hydrolysis of the ester. This simple procedure affords the (R)-(+)-acid with ee 70%,and recrystallization of the n-propylamine salt from ethyl acetate raises the ee to 100%. (16) Mason, S. F. In ‘Optical Activity And Chiral Discrimination”; Mason, S. F., Ed.; Reidel: New York, 1979; pp 1-24.
R1( R 2
R1= R 2
R1 ) R2
a View from the exo side of the molecule. The corresponding B,, transition dipoles should b e found at 8’ = 0 t 90”.
Scheme IV. In-phase (A) Coupling of the B,, Transition Dipolesa c3
a (a) 0 < 0 < -45”, repulsive interaction; (b) -45” attractive interaction.
< 0 < go”,
rivatives having C3 symmetry can be expressed as eq I, where indexes 1, 2, and 3 label each benzene ring and the asterisk indicate excitation. For each “monomer” transition, there are, thus, in $0
$A
=
XlX2X3
= (1/31’2)(xI*x2x3 + x1x2*x3 + xIx2x3*) $E
= (1/21’2)(xI*x2x3 - xIx2x3*)
$E = ( 1 / 6 1 ’ 2 ) ( 2 ~ ~ * ~ 2X~I 3X ~ *-XX~I X ~ X ~ * )(1) the “trimer” three excitations, two of which (E) are degenerate.
1302 J . Am. Chem. SOC.,Vol. 107, No. 5, 1985
Canceill et al.
Table I. Soecific Rotations of 2-12
[.Iz5,
concentration, compound 2 (M)-(+)
(PI-(-) 3 (MI-(+) 4
solvent a a
per 100 mL 0.1-0.3 0.1-0.3
deg'
589 +271 -272
578 +284 -285
546 +329 -329
436 +619 -619
365 +I160 -1162
error rangec, (h)
C,H analysis
4% 3%
C24H2406, C24H2406,
d d
C27H3006
(P)-(-)
a a
0.3 0.3
+293 -293
+306 -307
+354 -354
+667 -670
+1265 -1270
(W-(-)
a
0.25
-169
-177
-205
-386
-697
3%
C30H3009
-218
-223
-255
-473
-849
d
C33H3609
+17.7 -15.6
5% 10%
f f
5 (MI-(-)
a
0.1
6 (PI-(+)
(W+)
a a
2 1
7 (P)-(-)
a
8 (PI-(+)
lHZo H2°
+3.1 -3.4
+3.3 -3.4
+4.0 -4.0
0.2
-20.0
-21.0
-23.2
-47
-99
5%
C30H3606
(W-(-)
a a
0.7 0.7
+14.2 -14.1
+14.3 -14.1
+14.8 -14.8
+14.0 -14.4
-28.1 +23.2
d d
C33H4206
9 (M)-(-)
a
0.6
-47
-49
-56
-98
-155
d
C36H4806
10 ( M ) - ( + )
a a
0.25 0.25
+161 -165
+I68 -173
+192 -201
+354 -370
+630 -656
d 6%
C24H2403r '/2H2O
b b
0.25 0.25
+207 -199
+215 -208
+251 -241
+476 -456
+882 -846
d 4%
(P)-(-) 11
(W-(+) (P)-(-)
+8.2 -7.6
C21H1803~
2H20
12 (W-(+) a 0.5 +I90 +200 +227 +405 +685 d CnHxO, In spectrometric grade chloroform, stabilized with 0.5-0.8% ethanol. spectrometric grade dioxane. cMean deviation of the observed rotation of several samples obtained from different syntheses. dOnly one sample available, estimated accuracy f 2 % . CInthe case of 2, 10, and 11 which form crystalline hydrates, the rotations indicated in the table are not corrected for the water content. /See ref 4.
The splitting ( A t = 3V/hc) between the A and E components can be semiquantitatively evaluated by the point-dipole approximation, V = d12-3 [ p l y z- 3 d , (pl-dl2)(pZ~dl2)], ~ ~ where p1 and p 2 are the transition moment vectors, located at the centers of benzene rings 1 and 2, separated by dI2= 4.79 A.5 The theoretical rotational strengths R of the A and E coupling modes are evaluated as R,, = Im(( ~ o ~ f i ~ $ $m ,~fi)~$~,)](, where fi and fi are the electric and magnetic dipole moment operators, respectively. In an achiral derivative in which R, = R,, such as 1, the BZuand B1, transitions are polarized along the short and long axes of the aromatic rings, respectively, as sketched in Scheme IIIb, whereas in the chiral compounds the presence of substituents R I and R2 of different nature causes a rotation I9 of the transition moments with respect to the symmetrical positions (Scheme IIIa and c). The above expressions of the interaction potential and rotational strengths accordingly become eq I1 and 111, respectively, where is the angle
V = ( D m o n / d 1 2 3 ) (I9~cos2 ~ ~ 2@
+ y4cosz I9 sin2 0 - y4 sin2 0) (11)
RA = 31iz???d&,,oncos I9 sin I9 cos
a = -2RE
(111)
between the plane of each benzene ring and the C3axis (a = 43°),5 and D,,, = pu2is the dipole strength of the transition; the latter can be experimentally evaluated from the absorption (UV) spectrum of the trimer or, alternatively, of an appropriately substituted benzene ring (e.g., 4,Sdimethylveratrole (28) or guaiacol (29)) as the monomer model.37 (17) Mason, S. F. Inorg. Chim. Acfa Rev. 1968, 2, 89-109. ( 1 8 ) Platt, J. R. J . Chem. Phys. 1951, 19, 263-271. (19) (a) Sagiv, J. Tetrahedron 1977, 33, 2303-2313. (b) Ibid. 1977, 33, 2315-2320. (20) Collet, A,; Gottarelli, G. J . Am. Chem. SOC.1982, 104, 7383-7384. (21) Gottarelli, G.; Mason, S. F.; Torre, G. J . Chem. SOC.B 1970, 1349-1353. (22) Zgierski, M. Z.; Pawlinowski, M. Chem. Phys. L e f f . 1982, 93, 61 3-6 16. (23) While interactions with the E,, transitions are unlikely for the Bzu transition, this is no longer the case for Blu. This treatment, however, cannot account for the former transitions since it is based on the spectroscopic moment theory, which does not apply to allowed transitions; furthermore, the energy and polarization of the E,, components are not experimentally known. (24) Samori, B.; Mariani, P.; Spada, G. P. J . Chem. SOC.,Perkin Trans. 2 1982, 447-453.
The intensities of the B,, and B1, transitions of aromatic derivatives are currently interpreted in terms of the spectroscopic moments of the substituents,'*J9 and the rotation I9 of the electric dipole transition moment in C3-cyclotriveratrylenes should then depend on the nature of R, and R, as indicated in Scheme 111.
29
28
For the absolute configuration depicted, one obtains, when the spectroscopic moment of R, is greater than that of R, (R2 > Rl), a clockwise rotation (I9 > 0) which is inverted when R, < R,. Accordingly, the signs of the exciton CD bands should depend critically on the relative magnitude of the spectroscopic moments, the signs of I9 and of the interaction potential being the important factors in determining the actual sequence of the signs of the BZu and B,, couplets. As shown in Scheme IV, the in-phase A coupling of the Bzu transition moment vectors for 0 C I9 C 90' generates overall parallel electric and magnetic transition moments along (25) Gottarelli, G.;Samori, B.; Peacock, R. D. J . Chem. SOC.,Perkin Trans. 2 1977, 1208-1214. (26) Anderson, G. M.. 111; Kollman, P. A,; Domelsmith, L. N.; Houk, K. N. J . Am. Chem. SOC.1979, 101, 2344-2352. (27) Makriannis, A,; Fesik, S . J . Am. Chem. SOC.1982,104,6462-6463. (28) Schaeffer, T.; Laatikainen, R. Can. J . Chem. 1983, 61, 224-229. (29) Mersh, J. D.; Sanders, J. K. M.; Matlin, S . A. J . Chem. SOC.,Chem. Commun. 1983, 306-307. Dodziuk, H.; von Voithenberg, H.; Allinger, N. L. Tetrahedron 1982,38, 281 1-2819. See also: Lister, D. G.; Palmieri, P.; Zauli, C. J . Mol. Sfrucf.1976, 35, 299-302. Emsley, J. W.; Longeri, M.; Veracini, C. A,; Catalano, D.; Pedulli, G. F. J . Chem. Soc., Perkin Trans. 2 1982, 1289-1296. (30) Exner, 0. Collecf.Csechoslou. Chem. Commun. 1980,45, 843-853. (31) Brickell, W. S.; Mason, S. F.; Roberts, D. R. J . Chem. SOC.B 1971, 691-695. (32) Ballester, M.; Riera, J.; Suialter, L. J . Am. Chem. SOC.1964, 86, 4276-4282. (33) Shaw, J. E.; Kunerth, D. C. J . Org. Chem. 1974, 39, 1968-1970. (34) Lindsey, A. S. J . Chem. SOC.1965, 1685-1692. (35) Malcolm Bruce, J.; Sutcliffe, F. K. J. Chem. SOC.1956, 3824-3829. (36) The molar extinction c of the Bzutransition in guaiacol (29) is enhanced by a factor of ca. 1.8 on passing from acidic or neutral methanol (unionized phenol group) to a 0.2 N CH30Na solution (fully ionized); the corresponding increase in the band areas, however, is about 3 times. (37) Experimental dipole strengths and rotational strengths were evaluated from the UV and CD band areas: D,,, 91.8 X 10'o(l/X)~c dX and R 22.8 X 10-40(l/X)sAcdX, respectively.
-
-
J . Am. Chem. SOC.,Vol. 107, No. 5 , 1985
C3-Cyclotriceratrylene Derivatives Scheme V. In-phase (A) Couphng for the B,, Transition when 90" < 0 ' < -1 35" (Attractive) 32
I
1303
AE
V
the C, axis, hence positive rotational strength. However, the interaction potential V which is positive (repulsive) for small values of 8 (a), becomes clearly negative (attractive) for the larger ones (b); the "magic angle" 8, at which V = 0 is calculatedZoto be -45' from eq 11. It follows that the positive A component of the CD couplet should be found, in the first case, (a) at higher energy and, in the second case, (b) at lower energy, with inversion of the spectrum. Analogous considerations for the B,, transition are straightforward. While this simple picture of conservative CD adequately accounts for several of the experimental spectra (those of 2, 3, and 6-9), other derivatives (4, 5, and 10-12) show in the B2, region relatively large differences in the intensities of the two components, and in three cases (12 and ionized 2 and 11) only a single component is present. These differences can be semiquantitatively explained by taking into account interactions between different excitations in the three benzene rings.21.22 Restricting the interaction only to the B2, ( a )and B,, (p) transition^:^ the corrected A symmetry trimer state is expressed as eq IV, where the mixing coefficient X given by eq V can be calculated as the sum of the pairwise interactions between benzene rings undergoing different ( a or p) excitations. In the point-dipole approximation, the *A"
= ($Aa
+ X$Ap)/(l + X2)1/2
= (+A"IVI+A~)/(E,- Ep)
-32
x
I
I
240
290
nm
Figure 1. Theoretical sequence of exciton bands for 0 < 0 < -45O: (-) zero-order spectrum; (---) after configuration interaction. The CD curves were calculated by using the following data: Dmon(Blu)= 3 X cgsu and Dmon(B2J= 2 X cgsu, 0 = + 2 5 O ; these figures specifically correspond to ( P ) - ( + ) - 4 . I
I
I
(IV) (VI
interaction potential (eq V) becomes eq VI and is zero for 8 = 0 and 90°,being attractive between these values and reaching its maximum at 8 = 45'. The corrected rotational strength RALl(i) ($AaIVl$AP) = -(3/2d12)pl,~2psin 8 cos 8 ( 3
+ sin2 a)
(VI)
has the form of eq VII, where p and m are now the overall electric RAOL(i)= RA"
+ X(K,.mp + wp-m,) + XZRAp
(VII)
200
250
300
nm
+
Figure 2. Lower part: (-) isotropic absorption spectrum ( E l E , / 2 ) of an oriented sample of 1 in the nematic phase ZLI 1167; absorption components along the z axis (A,) and along the degenerate in-plane x and y axes (A, + AY) computed by the reduction method of ref 24, assuming a disk-like molecular shape. Upper part: (-) liquid crystal linear dichroism spectrum of the oriented sample (Ell - EL); reduced spectrum Ell- E , - ( d ' / 2 ) ( E , ,+ E , ) , showing the negative LD contribution of polarization along the z axis (in arbitrary units for d' = 0.28). (-..-.e-)
and magnetic transition moments of the trimer and RA" and RAP are the zero-order rotational strengths for the in-phase coupling mode of the B2, and B,, transitions, respectively, which can be evaluated from eq 111; since X
